1. Field of the Invention
This invention relates to a RF electron gun which is used for a linear electron accelerator and provided with a cathode activating device.
2. Description of the Prior Art
FIG. 1 is a sectional view showing a conventional electron gun described in, for example, "Nuclear Instruments and Methods in Physics Research" 222-226 pages, A272, 1988, North-Holland Physics Publishing Division. In FIG. 1, reference numeral 1 designates a RF electron gun, 2 a cathode which is provided at the center portion of the RF electron gun to generate electrons, 3 a filament for heating the cathode from the rear of the cathode, 4 a power source for the filament, 5 a baking control device which controls the power source for baking, 6 a RF cavity in which a field for accelerating electrons is formed, 7 a waveguide connected to the RF cavity, and 8 a microwave generating device connected to the waveguide.
Next, the operation of the RF cavity will be described. Before usage of the RF electron gun 1, the cathode 2 is heated for a long time while it is temperature-controlled by the filament 3, thereby performing baking. By this, gases such as impurities contained in the cathode 2 in manufacturing are removed by a vacuum pump not shown to suppress subsequent generation of ionized gases when the cathode 2 is heated in usage thereof. This allows the degree of vacuum in operation to be raised, by which it becomes easy to generate electrons from the cathode 2. In the above-mentioned literature, the degree of vacuum of 1 to 10.sup.-10 Torr is obtained by baking for several tens of hours at about 200.degree. C. This baking is used for lessening the reduction in thermionic electron radiation caused by formation oxides on the surface of the cathode depending on materials of the cathode 2 by residual gases. In practical usage, after baking, the cathode 2 is heated by the filament 3 to cause generation of electrons from the cathode 2. The electrons are subjected to an accelerating electric field of microwaves via the waveguide 7 from the microwave generating device 8 to cause acceleration. The principle of the acceleration will be described in detail in the embodiments of this invention presented herein.
Since the conventional RF electron gun has the above-mentioned structure, in order to control the temperature of the gun for a long time so that the temperature is maintained at a predetermined temperature for activating the cathode, there are problems that the apparatus for controlling and maintaining the predetermined temperature for a long time period is complicated and, difficult to operate, and requires a long time for the operation thereof.